QGIS » QGIS Application

Using Point data from a Postgresql table, Identify Feature reports "No feature at this position found", despite the cursor been placed in the middle of the symbol. This is regardless of whether the symbol is defined in map units (my preference) or millimiters.

However, using a different layer of point data in the same map (from a Postgresql table), Identify feature works as expected. I have been unable to work out what is the difference is between the two sets of data that would cause Identify Feature work in some cases and not others